A crystal oscillator includes: an oscillation circuit; first and second oscillation circuits connected to first and second temperature detection crystal units, respectively; a heating portion configured to constantly maintain an ambient temperature; a frequency difference detection portion that obtains, as a temperature detection value, “{(f2−f1)/f1}−{(f2r−f1r)/f1r}”, where “f1” and “f2” denote oscillation frequencies of the first and second oscillation circuits, respectively, and “f1r” and “f2r” denote oscillation frequencies of the first and second oscillation circuits, respectively, at a reference temperature; an accumulator that accumulates the temperature detection value; a rounding processing portion that performs rounding for the temperature detection value accumulated in the accumulator depending on a rounding factor designated independently from the accumulation number; and a heating control portion that controls power supplied to the heating portion based on the temperature detection value subjected to the rounding in the rounding processing portion.